[PATCH 0/1] nilfs2: fix data loss with mmap()

[PATCH 0/1] nilfs2: fix data loss with mmap()

[Ryusuke Konishi]

Andrew,

Please apply the following bugfix and send it to upstream.

It fixes a regression of nilfs2's mmap which was brought by the commit
136e8770cd5d1fe3 "nilfs2: fix issue of nilfs_set_page_dirty() for page
at EOF boundary".  Andreas Rohner recently found that the commit was
not perfect and causing reproducible silent data loss issue.

Thanks in advance,

Ryusuke Konishi
--
Andreas Rohner (1):
      nilfs2: fix data loss with mmap()

 fs/nilfs2/inode.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

[PATCH] nilfs2: fix data loss with mmap()

[Ryusuke Konishi]

From: Andreas Rohner <andreas.rohner@gmx.net>

This bug leads to reproducible silent data loss, despite the use of
msync(), sync() and a clean unmount of the file system. It is easily
reproducible with the following script:

----------------[BEGIN SCRIPT]--------------------
mkfs.nilfs2 -f /dev/sdb
mount /dev/sdb /mnt

dd if=/dev/zero bs=1M count=30 of=/mnt/testfile

umount /mnt
mount /dev/sdb /mnt
CHECKSUM_BEFORE="$(md5sum /mnt/testfile)"

/root/mmaptest/mmaptest /mnt/testfile 30 10 5

sync
CHECKSUM_AFTER="$(md5sum /mnt/testfile)"
umount /mnt
mount /dev/sdb /mnt
CHECKSUM_AFTER_REMOUNT="$(md5sum /mnt/testfile)"
umount /mnt

echo "BEFORE MMAP:\t$CHECKSUM_BEFORE"
echo "AFTER MMAP:\t$CHECKSUM_AFTER"
echo "AFTER REMOUNT:\t$CHECKSUM_AFTER_REMOUNT"
----------------[END SCRIPT]--------------------

The mmaptest tool looks something like this (very simplified, with
error checking removed):

----------------[BEGIN mmaptest]--------------------
data = mmap(NULL, file_size - file_offset, PROT_READ | PROT_WRITE,
		MAP_SHARED, fd, file_offset);

for (i = 0; i < write_count; ++i) {
	memcpy(data + i * 4096, buf, sizeof(buf));
	msync(data, file_size - file_offset, MS_SYNC))
}
----------------[END mmaptest]--------------------

The output of the script looks something like this:

BEFORE MMAP:    281ed1d5ae50e8419f9b978aab16de83  /mnt/testfile
AFTER MMAP:     6604a1c31f10780331a6850371b3a313  /mnt/testfile
AFTER REMOUNT:  281ed1d5ae50e8419f9b978aab16de83  /mnt/testfile

So it is clear, that the changes done using mmap() do not survive a
remount. This can be reproduced a 100% of the time. The problem was
introduced with the following commit:

136e877 nilfs2: fix issue of nilfs_set_page_dirty() for page at EOF
boundary

If the page was read with mpage_readpage() or mpage_readpages() for
example, then  it has no buffers attached to it. In that case
page_has_buffers(page) in nilfs_set_page_dirty() will be false.
Therefore nilfs_set_file_dirty() is never called and the pages are never
collected and never written to disk.

This patch fixes the problem by also calling nilfs_set_file_dirty() if
the page has no buffers attached to it.

Signed-off-by: Andreas Rohner <andreas.rohner@gmx.net>
Tested-by: Andreas Rohner <andreas.rohner@gmx.net>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: <stable@vger.kernel.org>
---
 fs/nilfs2/inode.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/nilfs2/inode.c b/fs/nilfs2/inode.c
index 6252b17..9e3c525 100644
--- a/fs/nilfs2/inode.c
+++ b/fs/nilfs2/inode.c
@@ -219,10 +219,10 @@ static int nilfs_writepage(struct page *page, struct writeback_control *wbc)
 
 static int nilfs_set_page_dirty(struct page *page)
 {
+	struct inode *inode = page->mapping->host;
 	int ret = __set_page_dirty_nobuffers(page);
 
 	if (page_has_buffers(page)) {
-		struct inode *inode = page->mapping->host;
 		unsigned nr_dirty = 0;
 		struct buffer_head *bh, *head;
 
@@ -245,6 +245,10 @@ static int nilfs_set_page_dirty(struct page *page)
 
 		if (nr_dirty)
 			nilfs_set_file_dirty(inode, nr_dirty);
+	} else if (ret) {
+		unsigned nr_dirty = 1 << (PAGE_SHIFT - inode->i_blkbits);
+
+		nilfs_set_file_dirty(inode, nr_dirty);
 	}
 	return ret;
 }
-- 
1.7.9.4

Re: [PATCH] nilfs2: fix data loss with mmap()

[Andrew Morton]

On Thu, 18 Sep 2014 23:56:25 +0900 Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp> wrote:

> From: Andreas Rohner <andreas.rohner@gmx.net>
> 
> This bug leads to reproducible silent data loss, despite the use of
> msync(), sync() and a clean unmount of the file system. It is easily
> reproducible with the following script:
> 
> ...
>
> --- a/fs/nilfs2/inode.c
> +++ b/fs/nilfs2/inode.c
> @@ -219,10 +219,10 @@ static int nilfs_writepage(struct page *page, struct writeback_control *wbc)
>  
>  static int nilfs_set_page_dirty(struct page *page)
>  {
> +	struct inode *inode = page->mapping->host;
>  	int ret = __set_page_dirty_nobuffers(page);
>  
>  	if (page_has_buffers(page)) {
> -		struct inode *inode = page->mapping->host;
>  		unsigned nr_dirty = 0;
>  		struct buffer_head *bh, *head;
>  
> @@ -245,6 +245,10 @@ static int nilfs_set_page_dirty(struct page *page)
>  
>  		if (nr_dirty)
>  			nilfs_set_file_dirty(inode, nr_dirty);
> +	} else if (ret) {
> +		unsigned nr_dirty = 1 << (PAGE_SHIFT - inode->i_blkbits);

It's quite cosmetic, but it is conventional to use PAGE_CACHE_SHIFT here.

Re: [PATCH] nilfs2: fix data loss with mmap()

[Ryusuke Konishi]

On Thu, 18 Sep 2014 12:17:08 -0700, Andrew Morton wrote:
> On Thu, 18 Sep 2014 23:56:25 +0900 Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp> wrote:
> 
>> From: Andreas Rohner <andreas.rohner@gmx.net>
>> 
>> This bug leads to reproducible silent data loss, despite the use of
>> msync(), sync() and a clean unmount of the file system. It is easily
>> reproducible with the following script:
>> 
>> ...
>>
>> --- a/fs/nilfs2/inode.c
>> +++ b/fs/nilfs2/inode.c
>> @@ -219,10 +219,10 @@ static int nilfs_writepage(struct page *page, struct writeback_control *wbc)
>>  
>>  static int nilfs_set_page_dirty(struct page *page)
>>  {
>> +	struct inode *inode = page->mapping->host;
>>  	int ret = __set_page_dirty_nobuffers(page);
>>  
>>  	if (page_has_buffers(page)) {
>> -		struct inode *inode = page->mapping->host;
>>  		unsigned nr_dirty = 0;
>>  		struct buffer_head *bh, *head;
>>  
>> @@ -245,6 +245,10 @@ static int nilfs_set_page_dirty(struct page *page)
>>  
>>  		if (nr_dirty)
>>  			nilfs_set_file_dirty(inode, nr_dirty);
>> +	} else if (ret) {
>> +		unsigned nr_dirty = 1 << (PAGE_SHIFT - inode->i_blkbits);
> 
> It's quite cosmetic, but it is conventional to use PAGE_CACHE_SHIFT here.

Agreed.

Thanks,
Ryusuke Konishi